Resistance to pseudorabies virus infection in transgenic mice expressing the chimeric transgene that represses the immediate-early gene transcription

Porcine Mx proteins inhibit pseudorabies virus replication through interfering with early gene synthesis

Pseudorabies virus (PRV) is an enveloped, linear double-stranded DNA herpesvirus that resulted in huge financial losses to the swine industry. In addition to vaccination, the development of antiviral molecules is also a beneficial supplement to the control of Pseudorabies (PR). Although our previous studies have shown that porcine Mx protein (poMx1/2) significantly inhibited the proliferation of RNA virus, it was unknown whether poMx1/2 could inhibit porcine DNA virus, such as PRV. In this study, it was investigated the inhibitory effect of porcine Mx1/2 protein on PRV multiplication. The results showed that both poMx1 and poMx2 had anti-PRV activities, which required GTPase ability and stable oligomerization. Interestingly, the two GTPase deficient mutants (G52Q and T148A) of poMx2 also had the antiviral ability against PRV, which was consistent with previous reports, indicating that these mutants recognized and blocked the viral targets. Mechanistically, the antiviral restriction of poMx1/2 came from their inhibition of the early gene synthesis of PRV. Our results for the first time shed light on the antiviral activities of two poMx proteins against DNA virus. The data from this study provide further insights to develop new strategies for preventing and controlling the diseases caused by PRV.

Alphaherpesvirus infection of mice primes PNS neurons to an inflammatory state regulated by TLR2 and type I IFN signaling

Pseudorabies virus (PRV), an alphaherpesvirus closely related to Varicella-Zoster virus (VZV) and Herpes simplex type 1 (HSV1) infects mucosa epithelia and the peripheral nervous system (PNS) of its host. We previously demonstrated that PRV infection induces a specific and lethal inflammatory response, contributing to severe neuropathy in mice. So far, the mechanisms that initiate this neuroinflammation remain unknown. Using a mouse footpad inoculation model, we found that PRV infection rapidly and simultaneously induces high G-CSF and IL-6 levels in several mouse tissues, including the footpad, PNS and central nervous system (CNS) tissues. Interestingly, this global increase occurred before PRV had replicated in dorsal root ganglia (DRGs) neurons and also was independent of systemic inflammation. These high G-CSF and IL-6 levels were not caused by neutrophil infiltration in PRV infected tissues, as we did not detect any neutrophils. Efficient PRV replication and spread in the footpad was sufficient to activate DRGs to produce cytokines. Finally, by using knockout mice, we demonstrated that TLR2 and IFN type I play crucial roles in modulating the early neuroinflammatory response and clinical outcome of PRV infection in mice. Overall, these results give new insights into the initiation of virus-induced neuroinflammation during herpesvirus infections.

Herpesviruses are major pathogens worldwide. Pseudorabies virus (PRV) is an alphaherpesvirus related to varicella-zoster virus (VZV) and herpes simplex virus type 1 (HSV1). The natural host is the pig, but PRV can infect most mammals. In these non-natural hosts, the virus causes a severe pruritus called the ‘mad itch’. Interestingly, PRV infects the peripheral nervous system (PNS) and induces a specific and lethal inflammatory response in mice, yet little is know about how this neuroinflammatory response is initiated. In this study, we demonstrated for the first time how PNS neurons tightly regulate the inflammatory response during PRV infection and contribute to severe clinical outcome in mice. Our work provides new insights into the process of alphaherpesvirus-induced neuropathies, leading to the development of innovative therapeutic strategies.

Virulent Pseudorabies Virus Infection Induces a Specific and Lethal Systemic Inflammatory Response in Mice

Pseudorabies virus (PRV) is an alphaherpesvirus that infects the peripheral nervous system (PNS). The natural host of PRV is the swine, but it can infect most mammals, including cattle, rodents, and dogs. In these nonnatural hosts, PRV always causes a severe acute and lethal neuropathy called the "mad itch," which is uncommon in swine. Thus far, the pathophysiological and immunological processes leading to the development of the neuropathic itch and the death of the animal are unclear. Using a footpad inoculation model, we established that mice inoculated with PRV-Becker (virulent strain) develop a severe pruritus in the foot and become moribund at 82 h postinoculation (hpi). We found necrosis and inflammation with a massive neutrophil infiltration only in the footpad and dorsal root ganglia (DRGs) by hematoxylin and eosin staining. PRV load was detected in the foot, PNS, and central nervous system tissues by quantitative reverse transcription-PCR. Infected mice had elevated plasma levels of proinflammatory cytokines (interleukin-6 [IL-6] and granulocyte colony-stimulating factor [G-CSF]) and chemokines (Gro-1 and monocyte chemoattractant protein 1). Significant IL-6 and G-CSF levels were detected in several tissues at 82 hpi. High plasma levels of C-reactive protein confirmed the acute inflammatory response to PRV-Becker infection. Moreover, mice inoculated with PRV-Bartha (attenuated, live vaccine strain) did not develop pruritus at 82 hpi. PRV-Bartha also replicated in the PNS, and the infection spread further in the brain than PRV-Becker. PRV-Bartha infection did not induce the specific and lethal systemic inflammatory response seen with PRV-Becker. Overall, we demonstrated the importance of inflammation in the clinical outcome of PRV infection in mice and provide new insights into the process of PRV-induced neuroinflammation.IMPORTANCE Pseudorabies virus (PRV) is an alphaherpesvirus related to human pathogens such as herpes simplex virus 1 and varicella-zoster virus (VZV). The natural host of PRV is the swine, but it can infect most mammals. In susceptible animals other than pigs, PRV infection always causes a characteristic lethal pruritus known as the "mad itch." The role of the immune response in the clinical outcome of PRV infection is still poorly understood. Here, we show that a systemic host inflammatory response is responsible for the severe pruritus and acute death of mice infected with virulent PRV-Becker but not mice infected with attenuated strain PRV-Bartha. In addition, we identified IL-6 and G-CSF as two main cytokines that play crucial roles in the regulation of this process. Our findings give new insights into neuroinflammatory diseases and strengthen further the similarities between VZV and PRV infections at the level of innate immunity.

Dominant-negative proteins in herpesviruses - from assigning gene function to intracellular immunization

Investigating and assigning gene functions of herpesviruses is a process, which profits from consistent technical innovation. Cloning of bacterial artificial chromosomes encoding herpesvirus genomes permits nearly unlimited possibilities in the construction of genetically modified viruses. Targeted or randomized screening approaches allow rapid identification of essential viral proteins. Nevertheless, mapping of essential genes reveals only limited insight into function. The usage of dominant-negative (DN) proteins has been the tool of choice to dissect functions of proteins during the viral life cycle. DN proteins also facilitate the analysis of host-virus interactions. Finally, DNs serve as starting-point for design of new antiviral strategies.

The first immunoglobulin-like domain of porcine nectin-1 is sufficient to confer resistance to pseudorabies virus infection in transgenic mice

Nectin-1 is an alphaherpesvirus receptor that binds to virion glycoprotein D (gD). Porcine nectin-1 mediates entry of pseudorabies virus (PRV), herpes simplex virus types 1 and 2 (HSV-1 and HSV-2), and bovine herpesvirus type 1 (BHV-1). The gD-binding domain of nectin-1 is the first or N-terminal immunoglobulin (Ig)-like domain of the entire ectodomain. Here, we generated three transgenic mouse lines expressing a fusion protein consisting of the first Ig-like domain of porcine nectin-1 and the Fc portion of porcine IgG1 to assess the antiviral potential of the first Ig-like domain of nectin-1 in vivo. All of the transgenic mouse lines showed significant resistance to PRV infection via intraperitoneal inoculation (survival rates of 67% to 100%). In the intranasal challenge, a lower but still significant protection was observed; 21% to 55% of the animals from the three transgenic mouse lines survived. The present results demonstrate that a soluble form of the first domain of porcine nectin-1 is able to exert a significant antiviral effect against pseudorabies virus infection.

Transgenic mice expressing a soluble form of porcine nectin-1/herpesvirus entry mediator C as a model for pseudorabies-resistant livestock

An approach to genetically engineered resistance to pseudorabies virus (PRV) infection was examined by using a transgene encoding a soluble form of nectin-1, also known as herpesvirus entry mediator C. Nectin-1 is an alpha-herpesvirus receptor that binds to virion glycoprotein D. Nectin-1 mediates entry of PRV, herpes simplex virus types 1 and 2, and bovine herpesvirus type 1. To assess the antiviral potential of an ectopic expression of the nectin-1 ectodomain in vivo, six transgenic mouse lines expressing a soluble form of nectin-1, consisting of an extracellular domain of porcine nectin-1 and the Fc portion of human IgG1, were generated. All of the transgenic mouse lines showed nearly complete resistance to PRV infection by means of both i.p. and intranasal routes. These results suggest that the introduction into farm animals of a transgene encoding a soluble form of nectin-1 would offer a potent biological approach to generating alpha-herpesvirus-resistant livestock.

Enhanced resistance to herpes simplex virus type 1 infection in transgenic mice expressing a soluble form of herpesvirus entry mediator

Herpesvirus entry mediator (HVEM) is a member of the tumor necrosis factor (TNF) receptor family used as a cellular receptor by virion glycoprotein D (gD) of herpes simplex virus (HSV). Both human and mouse forms of HVEM can mediate entry of HSV-1 but have no entry activity for pseudorabies virus (PRV). To assess the antiviral potential of HVEM in vivo, three transgenic mouse lines expressing a soluble form of HVEM (HVEMIg) consisting of an extracellular domain of murine HVEM and the Fc portion of human IgG1 were generated. All of the transgenic mouse lines showed marked resistance to HSV-1 infection when the mice were challenged intraperitoneally with HSV-1, but not to PRV infection. The present results demonstrate that HVEMIg is able to exert a significant antiviral effect against HSV-1 infection in vivo.

The latency-associated transcript promoter of pseudorabies virus directs neuron-specific expression in trigeminal ganglia of transgenic mice

The latency-associated transcript (LAT) promoter of pseudorabies virus (PRV) is unique among viral promoters in that it remains active in trigeminal ganglia during the latent state. It is not known which the viral or host proteins regulate expression of the PRV LAT gene in latently infected neurons. To determine whether host transcriptional proteins in neurons can regulate the PRV LAT promoter in vivo, three transgenic mouse lines containing the PRV LAT promoter (LAP; LAP1 and LAP2) linked to the chloramphenicol acetyltransferase (CAT) gene were generated. All of the transgenic mouse lines, in the absence of the viral proteins, displayed strong expression of the transgene in trigeminal ganglia in addition to other neuronal tissues such as cerebral cortex, cerebellum, hippocampus and olfactory bulb. Expression of the transgene in neurons of trigeminal ganglia was demonstrated by in situ hybridization. These data provide direct evidence that neuronal transcription factors are sufficient to activate the PRV LAP in vivo and that the promoter is neuron-specific.

Impaired development of the cerebellum in transgenic mice expressing the immediate-early protein IE180 of pseudorabies virus

Pseudorabies virus (PRV) infection in animals other than its natural host almost always gives rise to fatal diseases in the central nervous system as a result of infection of peripheral neurons and subsequently to the brain. PRV immediate-early protein (IE180) activates transcription of the PRV early and late genes, and other viral and cellular genes, and represses its own transcription. To examine specific effects of IE180 in neuropathogenicity, we have generated four transgenic mouse lines expressing IE180 in a tetracycline-regulated system. In the transgenic mouse lines, cerebellar symptoms such as ataxic gait, tremor and motor discoordination were observed. Histopathology of the cerebella in the transgenic mouse lines showing severe symptoms was remarkable for a failure of layer formation and a reduction in cerebellar size. These findings suggest that IE180 affects the cascade of gene expression for development of the murine cerebellum, resulting in the impairment of the cerebellar development and differentiation.

Analysis of regulatory functions for the region located upstream from the latency-associated transcript (LAT) promoter of pseudorabies virus in cultured cells

The latency-associated transcript (LAT) promoter of pseudorabies virus (PrV) is unique among the many promoters of the viral genome in that it remains active during the latent state. The regulatory mechanism of PrV LAT gene expression is complex and different between latency and lytic infection of cultured cells. Although two different sequences, LAP1 and LAP2, are thought to be involved in LAT gene expression, the function of the upstream region of the LAT promoter (LAP1 and LAP2) remains an enigma, even in cultured cells. To analyze the function of the upstream region, it is necessary to examine the effects of the upstream sequence on LAT gene expression in the absence of other viral proteins. Transient expression assays were performed by employing a series of reporter plasmids in which various sequences upstream of the LAT promoter (from nucleotide positions -592 to +423 relative to the transcriptional start site of the large latency transcript (LLT)) were linked to the chloramphenicol acetyltransferase (CAT) gene in cells of neuronal and non-neuronal origin. We identified a region (from nucleotide positions -3606 to -1386) that was capable of repressing the LAT promoter activity in Vero cells by analyzing CAT gene expression of the series of reporter plasmids. This effect was not observed in Neuro-2a cells. We have also shown that the LAT promoter activity of the reporter plasmid containing the upstream region was repressed by the immediate-early gene product IE180 in Vero cells, but not in Neuro-2a cells. These results suggest that the upstream region of the LAT promoter may have a role in repressing LAT gene expression in cultured non-neuronal cells.

Inhibition of pseudorabies virus replication by a dominant-negative mutant of early protein 0 expressed in a tetracycline-regulated system

Pseudorabies virus (PRV) early protein 0 (EP0) consisting of 410 amino acids is a transactivator of viral genes. A mutant consisting of amino acids 1-113 exhibits dominant-negative properties. In order to assess the antiviral potential of the EP0 mutant, Vero cells were transformed with the EP0 mutant gene expressed in a tetracycline-regulated system. The transformed cell lines showed marked resistance to PRV infection when expression of the EP0 mutant gene was induced. In the transformed cell line infected with PRV, synthesis of the immediate-early protein (IE180) and of EP0 was inhibited, whereas the levels of IE and EP0 messenger RNA (mRNA) were not decreased, as compared with those of the control cell line. The present results suggest that the EP0 mutant may not alter the efficiency of the viral gene transcription but rather translation efficiency of the viral mRNA.